My husband and I have started a bathroom remodel in our 80 + year old home. I've been lurking on this forum for a while and have learned a great deal from the information you all give, so thank you. Now for my problem, I want to trim a window inside of the shower with ceramic chair rail...In my dreams I simply would cut a 45 degree angle for the four corners and they would fit together perfectly and all would be peaceful and productive in our world. Reality is biting us in the b*tt, however. I will admit that I don't have a mathematical mind and that I'm sure I am missing something completely obvious and simple, but at the risk of looking like an idiot please take a look at the pics and tell me why the cuts aren't lining up. Each tile is 2x6 and I cut each tile at the 3 inch mark.
Thanks in advance for your advice.

Hi Kelly, Welcome to the forum.
I think one of the edges are longer than the other, but I guess you know that.
Is that an actual chair rail, or a sink rail (a sink rail is made to go along the edge of a counter top, so the backside is level in two spots, but the top edge goes further in to lap onto the counter)?
How did you determine where to cut the angle?

Hi Brad, thank you for the welcome!
It is chair rail molding so the back is level all the way across, except the curve. (its hollow) I just measured 3 inches in from whichever end I was cutting from, so that meant starting my angle in the middle of the tile. I was trying to wrap around the corner with 3 inches on either side. I just tried starting the angle at the corner, 3 inch wrap be damned, and it still comes out wrong. Maybe I'm just really tired!

Well Kelly, it's time to call it a day. Every tile setter knows that the proverbial wall is there, and that pushing past it only results in poorer quality (or one heck of a headache).
If you don't have a small speed square, pick one up at Lowe's or HD. They got cheap plastic ones for less than $5 I'd guess. Makes a huge difference when trying to make angled cuts.

Thank you all for taking a stab at this...I don't have a speed square, I was relying on the thing on my tile saw to make my 45. (I don't even know what its called ) That might be part of my problem. Is it recommended to cut from the center toward the corner or vice-versa?
Also, would it help to make it a compound miter? or am I just asking for more wasted tile?
Agree on that proverbial wall thing, I just think I'm this close to figuring it out...maybe I need a drink.

Starting at the corner usually works best, Kelly, but it all depends on the material and how your blade is cutting. You've created some scrap to work with , so try it both ways to see which works best. No need for a compound miter. Did you try placing them side by side as CX suggested? On occasion a tile might just have been made larger than its neighbor. Be sure to size them on the next run, as you use the speed square to mark the tiles or as a gauge on the saw.

Did you just hear the hallelujah chorus coming from the North? Cause that was me in Indiana...hubby decided not to take my word for it on the measurements and checked them himself. The tiles are actually 2 3/8 wide, so rather than making the cut at 3 inches he made it at 2 3/8, and it is perfect. I'm sure there is some mathematical equation for why it worked but I won't understand it. Thank you for your patience and expertise...this job has only just begun-I'll be back!
(I'm gonna finish my drink!)